Add events to all input devices

Fairly major tidy up of the hierarchy as well. There's now a trivial
base class: InputDevice which simply permits reading of state.
WaitableInputDevice descends from this and introduces waitable events
and callbacks, and provides a hook for calling them but needs further
machinery to activate that hook.

DigitalInputDevice (crap name?) descends from WaitableInputDevice and
uses the standard RPi.GPIO callback mechanisms to handle events. This is
intended for use with trivial on/off devices with predictably small
bounce times.

Next is SmoothedInputDevice (crap name?) which also descends from
WaitableInputDevice. This includes a background threaded queue which
constantly monitors the state of the device and provides a running mean
of its state. This is compared to a threshold for determining active /
inactive state. This is intended for use with on/off devices that
"jitter" a lot and for which a running average is therefore appropriate
or for devices which provide an effectively analog readout (like
charging capacitor timings).

MonitorSensor and LightSensor now descend from SmoothedInputDevice, and
Button descends from DigitalInputDevice. All "concrete" classes provide
event aliases appropriate to their function (e.g. when_dark,
when_pressed, etc.)

docs/index.md

@@ -72,7 +72,6 @@ Alternatively:
 
 ```python
 from gpiozero import LED
-from time import sleep
 
 red = LED(2)
 red.blink(1, 1)
@@ -118,7 +117,7 @@ from gpiozero import Button
 
 button = Button(4)
 
-button.wait_for_input()
+button.wait_for_press()
 print("Button was pressed")
 ```
 
@@ -127,44 +126,54 @@ Run a function every time the button is pressed:
 ```python
 from gpiozero import Button
 
-def hello(pin):
-    print("Button was pressed")
+def warning():
+    print("Don't push the button!")
 
 button = Button(4)
 
-button.add_callback(hello)
+button.when_pressed = warning
 ```
 
 ### Motion Sensor
 
-Detect motion:
+Detect motion and light an LED when it's detected:
 
 ```python
-from gpiozero import MotionSensor
+from gpiozero import MotionSensor, LED
 
 pir = MotionSensor(5)
+led = LED(16)
 
-while True:
-    if pir.motion_detected:
-        print("Motion detected")
+pir.when_motion = led.on
+pir.when_no_motion = led.off
 ```
 
 ### Light Sensor
 
-Retrieve light sensor value:
+Wait for light and dark:
 
 ```python
-from time import sleep
 from gpiozero import LightSensor
 
+sensor = LightSensor(18)
+
+while True:
+    sensor.wait_for_light()
+    print("It's light! :)")
+    sensor.wait_for_dark()
+    print("It's dark :(")
+```
+
+Run a function when the light changes:
+
+```python
+from gpiozero import LightSensor, LED
+
 sensor = LightSensor(18)
 led = LED(16)
 
 sensor.when_dark = led.on
 sensor.when_light = led.off
-
-while True:
-    sleep(1)
 ```
 
 ### Temperature Sensor
@@ -196,3 +205,4 @@ sleep(5)
 left_motor.off()
 right_motor.off()
 ```
+